Potential hazard of Earth-asteroid collisions to be discussed at international
space conference at Cornell July 26

ITHACA, N.Y. -- The possibility of the Earth being struck by comets or
asteroids is being given more and more attention by researchers, according
to Paul Chodas of NASA's Jet Propulsion Laboratory (JPL). Chodas will
discuss this potential threat to the planet when he moderates one of the
daily press conferences that will provide important new insights into the
latest space research at the seventh International Asteroids, Comets and
Meteors Conference at Cornell University July 26-July 30.

A schedule of the press conferences is available on the ACM media web site
http://www.news.cornell.edu/releases/July99/ACM/. The site provides the
latest press releases on research to be reported at the conference, as well
as a list of researchers from around the world who will be attending the
meeting.

Chodas's press conference (Tuesday, July 27, 10 a.m., 305 Ives Hall) will
cover asteroids and meteors collectively known as near-Earth objects
(NEO's). Other participants will include David Rabinowitz of JPL, Richard
Binzel of the Massachusetts Institute of Technology, Alan Harris of the
Jet Propulsion Laboratory (JPL) and Andrea Milani of the University of
Pisa, Italy.

In recent movies, asteroids and comets are shown threatening to collide
with the Earth, only to be destroyed at the last minute by astronaut
heroics. But Chodas believes that the hazards of collisions with comets
or asteroids are more than a topic for fiction. Chodas and Milani will
discuss their independent efforts to predict close-Earth approaches and
impact probabilities further into the future than has been previously
possible.

It has been estimated that only 15 to 20 percent of NEO's larger than one
kilometer have been detected to date. Several telescopic discovery
programs are actively searching for these large NEO's, and the discovery
rate is increasing. However, Chodas believes, it may be at least 10 years
before 90 percent of the total population is revealed.

A new analysis by Rabinowitz shows that there may be only half as many
large hazardous objects as previously estimated. Harris will present new
evidence to refute a controversial theory that the Earth is continually
bombarded by a population of house-sized comets.

What is the nature of these seemingly "loose cannons" that might be posing
a threat? A new view has evolved, suggesting that many large asteroids
are "rubble-piles," according to William Bottke, a research associate in
Cornell's Department of Astronomy, who will moderate the press session
(Friday, July 30, 10 a.m., Princeton-Yale Room, Statler Hotel) on asteroid
moons and spins. Other participants will include Petr Pravec of Ondrejov
Astronomy Institute, Czech Republic, and JPL's Harris.

Until recently, most planetary scientists considered asteroids to be little
more than beat-up rocks in space, with solid interiors and lunar-like
surfaces. It is difficult to prove or disprove this theory since remote
sensing techniques can only probe the top layers of an asteroid's surface,
says Bottke. But data taken from spacecraft fly-bys, ground-based
observations and computer modeling indicate that large asteroids are no
more than piles of rubble held together not by physical strength but by
the gravitational attraction of the pieces of rubble.

Although Pravec's research, based on an extensive study of near-Earth
asteroid (NEA) rotation rates, is consistent with the "rubble-pile"
scenario, his new results suggest that some small asteroids are solid.
At the conference he will discuss two asteroids, 1996 KY26 and 1995 HM,
both smaller than 100 meters, that are spinning so fast that they have to
be solid objects -- weakly bound rubble piles fly apart if spun too fast.
These two bodies are small enough that they provide the "missing link"
to the rubble-pile theory that astronomers have been looking for --
filling the gap between between kilometer-size asteroids that cannot
spin overly fast and fast-spinning meter-sized meteorites that are known
to be solid.

Computer modeling suggests the transition size for solid objects to turn
into rubble piles is roughly a few hundred meters in diameter. More support
for the rubble-pile theory also comes from Pravec: He will report that many
NEO's have small moons. These moons are most likely to be a by-product of
close encounters between rubble-pile asteroids and planets. When a
rubble-pile asteroid passes too close to a planet like Earth, tidal forces
can pull it apart, leaving some of the fragments to orbit one another.

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